Preparation of New Catalytic Materials by Small Particle Technology

Doctoral thesis, 1997

New techniques for preparation of catalytic materials with novel properties are presented in this thesis. Three new catalytic materials; washcoated fibres, highly dispersed metal oxides on titania coated silica particles and discrete colloidal ZSM-5 crystals with a low SiO2/Al2O3 ratio have been prepared and characterised. The catalytic properties of cobalt oxide and ceria promoted noble metals, and Cu-ZSM-5 catalysts made of commercial and discrete colloidal ZSM-5 crystals have been evaluated. Porous coatings were applied to alumina and silica fibre substrates by a stepwise procedure comprising charge reversal of the fibres, electrostatic adsorption of sol particles and fixing of the sol particles onto the fibre surface. Charged polymers proved to be excellent for charge reversal of fibre substrates owing to low sensitivity to dilution, whereas charge reversal was more difficult to accomplish with low molecular weight agents. The role of the charge reversing agent in the coating of the fibre substrates is discussed. Oxides of cerium(IV), iron(III), chromium(III), manganese(II), tin(II), nickel(II) and cobalt(II) and sulfides of nickel(II) and cobalt(II) were highly dispersed on the surface of TiO2/SiO2 particles with a particle size in the range 80-500 nm by a stepwise procedure. The mechanisms of controlled deposition of metal hydrous oxides on substrate particles are discussed. Discrete colloidal ZSM-5 crystals with a SiO2/Al2O3 ratio in the range 40-80 were prepared in a joint project by autoclaving an aluminosilicate gel at 120-160 °C formed upon mixing of TPAOH containing silicate and aluminate solutions. A high synthesis temperature was found to favour the incorporation of aluminium in the zeolite structure. The size of the crystals increased with decreasing SiO2/Al2O3 ratio. Catalysts made of discrete Cu-ZSM-5 crystals exhibited a higher NO-reducing ability in oxygen excess atmosphere compared with catalysts made of commercial Cu-ZSM-5 crystals of the same SiO2/Al2O3 ratio and copper exchange level. The low temperature oxidation of CO and propene over alumina supported Pt and Pd was significantly affected by the reactant gas stoichiometry, promoting oxide and pretreatment atmosphere. Prereduced cobalt oxide exhibited a light-off performance that resembles those normally observed over noble metal catalysts, whereas preoxidised cobalt oxide performed as regular catalytically active metal oxides. The support particles (Al2O3,TiO2/SiO2) were found not to affect the light-off performance of prereduced Pt/Co3O4 catalysts under the experimental conditions used. A mechanism to explain the remarkable low temperature activity over prereduced cobalt oxide is suggested.